Cassettes and boats are used as wafer carriers to transport semiconductor wafers in the various processes involved in the manufacture of said semiconductor wafers. Where low temperature processes are involved, it is common practice to use plastic or other lower cost wafer carriers and where high temperature processes must be endured, quartz or other high temperature materials are used. The lower cost plastic carriers are more "friendly" to the wafers in that they are softer and protect the wafers better against physical damage. This is especially true where the wafers are subjected to washing and cleaning processing cycles.
"Cassette to cassette" systems involve more than one kind of wafer carrier, for example, teflon carriers may be used for washing or cleaning operations and quartz boats may be used for oven processing. Other materials are also used for wafer carrier fabrication, depending upon the particular application.
Transfer machines are used to transfer wafers from one carrier type to another and back again. The transfer machine handles the wafers in lots of, say, twenty-five, removing them from one carrier, while a human operator manually removes the input carrier and replaces it with the output carrier, and then replacing them in the output carrier.
Of course it is advantageous to eliminate the human handling function because of the possibility of wafer or carrier breakage with its attendant high cost and because manufacturing efficiency is better served if the human operator is not required. Furthermore, in many oven systems, it is necessary to locate the boat in the oven input area with some precision.
Until recently, there were no three axis robotic systems that had the accuracy and repeatability which would allow such cassette to cassette operations to be automated. With the advent of a relatively low cost, high resolution three axis robotic system which is capable of extreme accuracy, high speed and programmable track, velocity and acceleration characteristics, such as the Model CCR 3000 manufactured and sold by Wollmann Engineering, Inc., 202 South River Drive, Tempe, Arizona 85281, it has become economically and physically feasible to handle and transport wafer carriers with a high degree of accuracy, speed and safety. The CCR 3000 is specified at a useful accuracy of plus or minus 0.004 inches (measured performance is even better) which allows the use of tight tolerance engagement and pick up interfaces.
FIG. 1 shows prior art wafer carrier 10 of a plastic type as manufactured by Fluoroware, 102 Jonathon Boulevard, Chaska, Minnesota 55318. A parallel two rod 12 pickup mechanism of prior art design is inserted into slots 14a and 14b to engage carrier 10. Rods 12 must be relatively small in diameter to fit within the space provided by slots 14. Because of the small diameter of rods 12, they are not as structurally stiff as might be desired. Note that engagement travel 16 of pickup rods 12 must be equal to or greater than the length of rods 12. This necessarily long travel distance represents longer operating time and higher cost. Furthermore, the engagement travel is in the z-axis of the three axis robot which drives the engaging mechanism. The z-axis travels horizontally and perpendicular to the loading station (parallel to the wafers) and intrudes upon an operator work and walk area in front of the loading area.
Since the entire mechanism is in a very costly clean room environment, it is important that no excessive use of floor area be consumed by the robot operation. That means that the more the z-axis arm intrudes into the work area, the more danger there is that an operator may be injured by the robot z-axis arm or that the operator will jostle the robot arm and cause damage to wafers, carriers or both. A carrier loaded with wafers may be worth many thousands of dollars. Damage to such a load can be very costly.
Furthermore, the clean room represents very expensive floor space because of the extreme costs of maintaining its purity. Any excursion of the z-axis arm beyond the footprint of the loading station represents an additional volume which must be provided and maintained as a clean environment. For that reason, it is especially advantageous to minimize the excursion of the z-axis robot arm outside of the loading station.